Azeotropic composition



United States Patent ABSTRACT OF THE DISCLOSURE The disclosure relates to azetropic mixtures of tetrachlorodifluoroethane and isopropanol.

The present application is a continuation-in-part application of US. Ser. No. 677,73 8, filed Oct. 24, 1967, which in turn is a continuation-in-part of U8. Ser. No. 590,227, filed Oct. 28, 1966 both of which are abandoned.

This invention relates to an azeotropic composition and particularly to the azeotropic mixture of tetrachlorodifluoroethane and either one of the compounds ethanol, isopropanol, n-propanol, methyl ethyl ketone or methyl n-propyl ketone.

Several of the chlorofiuoroethanes have attained widespread use as specialty solvents in recent years, particularly tetrachlorodifluoroethane. This is a relatively high melting compound (CCl FCCl F, 245 C.) which is nontoxic and nonfiammable, and which has satisfactory solvent power for greases, oils, waxes and the like under certain conditions. It has therefore found widespread use in cleaning electric motors, compressors, oxygen storage tanks, photographic film, lithographic plates, typewriters, instruments, gauges, sound tape, and as non-corrosive brines.

For certain solvent purposes however, the chlorofluoroethanes alone have insufficient solvent power. This is particularly true in the electronic industry during the manufacture of printed circuits Printed circuits are well known in the electronics art; and consist of a circuit formed from a soft metal on a solid, non-conducting surface such as a reinforced phenolic resin. During manufacture, the solid surface is coated with the metal, the desired portion of metal is coated with an impervious coating, and the excess metal is removed by etching with a suitable acid. Afterthe excess metal has been removed, it is necessary to remove the impervious coating because solder joints must be made to the printed circuit and these will not form if the coating is present. After the impervious coating is removed, the circuits are coated with a rosin flux to permit the joints to be soldered, after which the rosin flux'must then be removed.

The chlorofluoroethane solvent does not have suflicient solvent power to clean printed circuits; that is, to effectively remove the rosin flux. Although mixtures of solvents may be used for this purpose they have the disadvantage that they boil over a range of temperatures and consequently undergo fractionation in vapor degreasing or ultrasonic applications which are open to the atmosphere. When employing either of these methods the solvent must also be substantially nontoxic and nonflammable for safety reasons.

Tetrachlorodifluoroethane is a relatively high boiling fluorocarbon and for this reason especially advantageous in vapor degreasing applications since at these temperatures the hot vapor has more of a tendency to dissolve high melting greases, or fluxes as well as oil residues and the like. When articles such as circuit boards are passed "ice through a vapor degreaser, the solvent vapors tend to condense on the article until the articles are heated by the vapors from room temperature up to the temperature of the vapor. The condensation thus formed on the articles tends to drip back into the solvent reservoir taking with it some of the soil on the article. For this reason the ability of a cleaning solvent to condense on the surface is especially advantageous. Higher boiling solvents prolong this condensation effect in a continuous degreaser since it takes a greater amount of time to bring the article passing through the degreaser up to the vapor temperature of the solvent. Consequently higher boiling solvents generally have better cleaning power per unit of time in a continuous vapor degreaser than the lower boiling solvents.

Tetrachlorotrifiuoroethane also is a better solvent than materials such as trichlorotrifiuoromethane however, it sufliers the disadvantage that it is solidus at room temperature whereas the latter is liquidus. Accordingly, tetrachlorodifiuoroethane is more difiicult to handle than liquid type cleaning solvents.

It is an object of this invention to provide a constant boiling or azeotropic solvent that is liquid at room temperature, will not fractionate and also has the foregoing advantages. Another object is to provide an azeotropic composition which is valuable as a solvent and particularly for cleaning printed circuits. A further object is to provide an azeotropic composition which is both relatively nontoxic and nonfiammable both in the liquid phase and in the vapor phase and which at the same time is an excellent solvent for cleaning printed circuits especially by means of a continuous vapor degreasing machine.

The above object of this invention may be accomplished by a novel azeotropic composition of tetrachlorodifluoroethane (e.g. 1,1,2,2-tetrachloro 1,2 difluoroethane) and either one of the compounds ethanol, isopropanol, n-propanol, methyl ethyl ketone or methyl-n-propyl ketone. Although 1,1,2,2-tetrac:hloro 1,2 difluoroethane is a preferred tetrachlorodifluoroethane, the isomer l,1,l,2-tetrachloro-2,2-difluoroethane may be substituted therefor in whole or in part and especially in minor amounts or trace amounts. All of these mixtures form azeotropes which distill at a constant temperature, the liquid phase and the vapor phase in. equilibrium therewith having the same composition. Such mixture is relatively nonfiammable and nontoxic in both the liquid phase and the vapor phase. These mixtures are particularly useful as solvents for greases, oils, waxes, and the like and cleaning electric motors, compressors, photographic films, lithographic plates, typewriters, precision instruments, gauges, sound tapes, and the like and are particularly useful for cleaning printed circuits.

Although the azeotropic mixtures are obtained at approximately 760 mm. Hg a variation in pressure and consequently a change in the compositions and boiling points are also intended to be within the broad scope of the invention. Thus the azeotropes may contain many different proportions of all of the aforementioned components provided a constant boiling mixture is obtained at the various pressures at which the compositions are used. Stated otherwise any pressure may be employed to obtain the aeotropes of this invention as long as a two component constant boiling mixture is obtained, and accordingly the ratio of components of the azeotropes of the invention will also vary. The variation of components is thus within the skill of the art and is easily determined once it is known that the halogenated hydrocarbons of this invention will form the aforementioned azeotropes. In a preferred embodiment the present invention relates to the aforementioned azeotropes that boil at atmospheric pressure about 25, especially about 15 mm. Hg.

3 EXAMPLE I Composition, Boiling point,

percent by C. -760 mm.

Example Component weight Hg.

1 1,l,2,2-tetrachloro- 75. 6

1,2-difiuoroethane Ethanol 24. 4 71 EXAMPLES II-V The method of Example I is repeated using different mixtures of alcohols and ketones, the results of which are as follows:

screen method. The excess copper is then removed by etching with a ferric chloride-hydrochloric acid bath, sometimes containing ammonium chloride, leaving on the board the copper that is covered by the ink. After washing oil? the etch solution, the asphalt ink is removed by cleaning with the azeotropic composition ofthis invention in an ultrasonic bath (some mechanical scrubbing is often used). The entire surface of the board is coated with a rosin flux and dried. The electronic components (resistors, capacitors, etc.) are then added at the proper places for soldering to the circuit. The .board is then passed over amolten solder bath, contacting the desired joints with the molten metal, whereby the soldering is effected. After cooling, the excess rosin flux remaining on the board must be removed since, if present in the final assembly, it will lead to corrosion, poor electrical resistance and other deleterious properties.

The board is cleaned by placing it in an ultrasonic bath of any of the aforementioned azeotropes and operating at about 32 kilocycles per second at about 38 C., where it remains for about one minute.

When the board is cleaned with the azeotropic mixtures COMPONENTS, PERCENT BY WEIGHT tetrachloro- Methyl Example No. ethane n-Propanol l-Propanol ketone Boiling point,

Methyl C. at n-propyl approximately ketone 760 mm. Hg.

1,1,%2-tetrachloro-1,2'difluoro- 75.6 i about 10% especially 5: about 5% et ane.

Ethanol 24.4 :l: about 10% especially :l; about 5% 1,1,2,Ztetrachloro-l,2-difluoro- 86.5 :1: about 10% especially :l: about 5% ethane.

n-Propanol 13.5 =1: about 10% especially :l: about 5% 1,1,2,2-tetrachloro-1,2-difiuoro- 72.2 =1: about 10% especially about 5% ethane.

i-Propanol 27.8 :b about 10% especially =1: about 5% 1,l,2,2-tetrachloro-1,2-difluoro 14.0 5; about 10% especially =1; about 5% et ane.

Methyl ethyl lactone 86.0 about 10% especially i about 5% 1,1,2,2-tetrachlro-l,2difluoro- 93x :1: about 10% especially =1; about ethane. Methyl n-propyl lactone 7 :l: about especially i about 5% Thus where the azeotrope contains 75.6 parts by weight of 1,l,2,2-tetrachloro-1,2-difluoroethane about 10% as noted above, it is intended that this component may vary from about 68.1 to about 83.1 parts by weight and so forth for the rest of the ranges of all the other above components and their equivalents.

Printed circuit boards are usually prepared by impregnating glass cloth, nylon, or paper laminates with a phenolformaldehyde resin or an epoxy resin. Printed circuits are prepared by a variety of methods. In a typical procedure, the board consists originally of a phenolic resin impregnated base to which is bonded a sheet of copper, 2 to 4 mils thick, covering one surface of the board. The desired circuit is drawn on the copper With an asphalt based ink using the silk of this invention substantially all of the rosin flux is re moved without any detrimental eifect on the board which constitutes the backing of the printed circuit.

Although the invention has been described by reference to some preferred embodiments it is not intended that the broad scope of the novel azeotropic compositions be limited thereby but that certain modifications are intended to be included within the spirit and broad scope of the following claims.

What is claimed is:

1. An azeotropic composition consisting egsentially of about 72.2 parts by weight of 1,l,2,2-tetrachl0ro-l,2- difluoroethane and about 27.8 parts by weight of 'isopropanol.

2. A method for cleaning a solid surface comprising contacting said surface with the composition of claim 1.

References Cited UNITED STATES PATENTS 3/1969 Bauer et al. 252172 

